Thermal circuit breaker



Ptemed a. 17, 1939 UNITED sTATs 'rnEnMAL cIaoUrr BREAKER Calvin J. Werner, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich.,u a corporation of Delaware Application February 14, 1938, Serial No. 190,437

' 3 Claims.

This invention relates to thermal responsive switches and includes among its objects the provision of a switch of such 'construction that, inl

contacts so that sparking will be reduced to a minimum.

Further objects and advantages of the present m invention will be apparent from the following description, reference being had to the accompanying draWings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings: Flg. lis a perspective view of a switch embodying the present invention, the switch being closed. Fig. 2 is a longitudinal sectional view of the switch showing the contacts closed.

Fig. 3 is a view similar toFig. 2, showing the 20 contacts opened.

Fig. 4 is a chart illustrating the operation of the present switch.

Fig. 5 is a diagram of certain movable parts of the switch. 1 J 25 The switch comprises a nonconducting bracket or imounting base 20, supporting terminals 2| and 22 having squared shankvpo'rtions 2|a and 22a which fit into square holes in the plate 20. The bolt 2| passes through sleeves 23 and 24 and 30 threadedly engages a nut 25. The sleeve 23 telescopically receives the sleeve 24. Between shoulders provided by the sleeves 23 and 24, are clamped a. contact stop 25 of relatively stiff' metal and a relatively flexible contact spring 21 made of suitable resilient and electrically conducting material, such as phosphorous bronze. The contact spring' 21 has a center portion or hub 21a which is apertured to receive the sleeve 24 by which the hub 21a is clamped against the contact stop 40 23. 'From the hub 21a extends an ear'21b which is apertured to'receive a button 26h which is deformed from the contact stop 26. In this way the spring 21 is prevented from turning with respect to the contact stop 26. The contact stop 45 23 is prevented from turning with respect to the 'plate 20 by providing the plate 20 with a notch 20a which receives a strip 26a integral with the stop. Obviously, tightening the nut 25 on the bolt 2| secures 'contact stop 26 and the contact 50 spring 21 inthe des'ired position.

o Strip 21 carries a contact 28 having a portion 23a of larger diameter than the 'adjacent hole in thestop 26,,which hole is larger in diameter than' the portion 28b of contact 23; and'the portion 55 23h is wider 'axially than the thickness ofl the stop 26. The contact 28 has a rivet shank portion 280 which passes through the spring 21 and a washer 29 and which is riveted. as shown, to secure the contact 28 -to the spring 21. That portion of spring 21 which carries the contact 23 5 is connected with the hub portion 21a of the spring by resilie'nt. arms 21c which are so biased that the spring 21 vis urged downwardly against the stop '26, as shown in Fig. 3.

Contact 28 is positioned for eng'agement by a contact 29 attached to the free end of a bimetallic thermdstat blade 30, the fixed end of which is clamped between sleeves 3| and 32 which su-rround the bolt 22 whichreceives a nut 33 by which,

in cooperation with the bolt 22, the parts 30, 3| and 32 are clamped in the position shown. In order to prevent turning ofthe blade 30 with respect to the bolt 22 and hence to prevent shift ing of the blade with respect to the plate 20, the blade is provided with a downwardly extending' strip 30a received by notch 35 in the plate 20. This strip 30a may serve as a means for attaching a Wire, if desired.

The contact 29 is biased toward the contact 23 by coiled tension spring 40 having one end anchored to a strap 4| which encircles the sleeve 3| and is received by groove 3|a therein. The other end of the spring 40 engages the yoke 42a of a U-shaped wire link member 42, the branches 42h of which are -each pivotally connected at 42c with anear 30h integral with the blade 30.

Referring to Fig. 2 it will be noted that the action line of spring 40, referring to the imaginary line which joins the two points of spring anchorage, is substantially below the pivotal connection between the link 42 and the thermostat blade ears 30h. Therefore, in normal condition, the spring of the switch operates to exert a substantial force ,approximately vertically as viewed in Flg. 2 so as to bias the contact 29 upwardly against the 40 contact 28 with contact pressure suflicient to cause the portion 28a,of the contact 28 to bear against the under surface of the contact stop 23 and to urge the spring 21 upwardly away from the stop 26. 4 p 45 As the temperature of the blade 30 increases above a certain base or room temperature due to increase of temperature of the. immediate surrounding atmosphere, or due to the heating eifect of an electric current fiowing through the blade, the blade will tend to bend downwardly in opposition to the force exerted against it by the spring 40. When the temperature of the bimetal blade 30 has reached a predetermined switch opening temperature, the tendency of the blade to bend move it will continue to move more and morev rapidly away from the contact 28 because the component of force exerted by the spring 40 against the bimetal 30 diminishes since the pivot 42c is moving toward the center line of'action of the spring 40. This force component of spring dirninishes at a rate substantially greater than the rate of decrease of the force of reaction exertecl by the bimetal strip against the spring 40. Therefore once the contact 29 starts to separate from the contact 28, separation takes place in a raDidly accelerating manner and, hence,.abruptly in order that sparking between thel contacts will be minimized.

As the contact 29 starts to move downwardly the contact 28 follows up for ashort distance until the contact spring 27 engages the upper side of the contact stop 26. This slight amount of follow-up movement is sufllcient to be sure that the contact 29 has started downwardly a sufficient distance to insure that it will move all the way downward to separate the contacts. Theoretically such follow-up movement is not required; but, as a practical matter owing to inaccuracies in manufacture and friction between the moving parts, a fiuttering action might occur if separation of the contacts were to take place during the first small increment of the downward movement of contact 29.

Mention has been made of the fact that the upward component of the force exertedby the spring 40 against the end of the bimetal blade 30 diminishes at a greater rate than the rate of decrease of the reaction of the blade 30 against the spring 40. This phenomena can best be explained with reference to Figs. 4 and 5. Fig. 5 diagrammatically represents spring 40, blade 30 and link 42 in cooperative relation when the contacts are closed. In this figure the dot 3| represents the bushing 3| to which the spring 40 and the blade 30 are attached. Dot 42c in Fig. 5 represents 'the pivotal connection between the spring-loaded arm or link 42 and the blade 30. Dot 42d represents the pivotal connection between spring 40 and the spring-loaded arm 42. The chart shown in Fig. 4 is based on certain dimensions of the part-s. For example, the bladel 30 is regarded as a cantilever spring 1.583 long, the arm 42 is .5375" log and the distance between the eyes of spring 40 is 1.956". Normally the spring eye indicated at point 42d is .040 from the plane of the dots 42c and 3|`y which diagrammatically represents the plane of the blade 30. Normally the dot 42c representing the pivotal connections '42c in Figs. 1, 2 and 3 is .032" from the line of action of spring 40.

In Fig. 4 the horizontal line X-X is the base or zero line of spring-leading; Line Y-Y is the zero or 'base line of toggle de`fiection. Zero distance horizontally from line Y-Y represents the condition when the pivot point 42c coincides with the center line of action of spring 40, that is, the toggle pivot is in neutral position. Measurements horizontally to the right of line Y-Y represent the distance from the toggle pivot below the line 40. Distance vertically above the line X-X represents the reaction of the bimetal blade 30 against the Vertical component of the' springloaded arm 42 due to stress in the bimetal blade. Distance vertically below the line X-X represents reaction of the bimetal blade against the contact 28 due to Stress in the bimetal blade. Distance vertically above the line X X also represents the upward component of the spring-loaded arm 42 against the bimetal blade 30.

In Fig. 4 line a-a' is the load-defiection curve of the bimetal blade 30 at room temperature which, for example, may be around F. Line b--b' is the load-deflection curve of the blade 30 at reset temperature which may be, for example, 167.5 F. Line c-.c' is the ioad-deflection curve of the blade 30 at trip temperature, which may be, for example, at 201o F. Line c-e shows the load-defiection characteristic of the Vertical component exerted upon the blade 30 by the springloaded arm 42, When the contacts are closed and the blade is at room temperature, the effective reaction exerted by the blade 30 against'the toggle pivot 42c is zero, as represented by the point a. The defiection of point a from the toggle neutral is .033", which agrees with the dimensions shown in Fig. 5. The reaction of the Vertical component to the spring-loaded arm 42 against blade 30 is represented by point c which is 3.41 ounces. This represents the contact pressure exerted upwardly by contact 29 against contact 28 when the switch is in the position shown in Fig. 2.

As the temperature of the bimetal blade 30 increases there is progressively built up in it a force opposing the reaction of the spring-loaded arm 42 which will cause downward movement of the pivot 42c to take place when the bimetal reactiondownwardly becomes greater'than the reaction of the spring-loaded arm upwardly. The characteristic of bimetal blade 30 at trip temperature is represented by line c-c' which intersects the line C-E at point C. At trip temperature the downward force of the bimetal blade slightly exceeds 3.41 ounces, thereby causing the contact 29 to begin to separate from the contact 28. As this separation takes place the downward reaction of the bimetal blade against the pivot 42c diminishes 'according to the line c-.

while the upward component of the force exerted by the spring 40 on pivot 42c diminishes along the line c-e, which has a steeper slope than the line c--, thereby denoting that the Vertical component of the force exerted by spring 40 diminlshes at a more rapid rate than the force exerted by the bimetal. Therefore, once the contact 29 begins to move downwardly from the position shown in Flg. 2 to that shown in Fig.. 3, the force differential increases from zero at point C to a value represented by line -e, when movement vstops due to the fact that the rivet button 29a on the lower side of the blade 30 touches the upper side of the spring 40, When the button`29a is in its lowermost position as indicated by point the link pivot 42c will be .015" above the line of action of spring 40. The line -e represents the trip line or the "off" position of the switch. After the switch has tripped open the value of which is the force exerted by the blade 30, is 2.75

ounces and the upward force exerted by the spring-loaded arm 42 represented by e is 1.60 ounces. 'I'herefore it is apparent that the force differential between the bimetal blade 30 and the spring-loaded arm 42,V has increased from. zero to 1.15 ounces during the tripping of the switch.

When the blade cools to a temperature slightly below that of 201 F. at which temperature the bimetal has the characteristic represented by line b-b', the switch will automatically close or reset and the closing of the switch will also take place by a snap action. This is due to the fact that, as the point 42c starts to ascend from the action line' of spring III, that is, to increase its distance from 0.15 to the maximum which is .033", the Vertical component of the force exerted by the spring 40 upon the bimetal blade 30 as represented by line e-c increases at a rate, substantially greater than the rate of increase of the reaction of the bimetal blade against the arm l! as represented by the line e-b.

From the foregoing description of the construction and modeof operation of the temperature responsive snap switch it 'is apparent that the switch comprises a bimetal blade and a control spring in opposing relation and connected with each other by means of a link or spring-loaded arm, the relation of these parts being such that separation of the switch contacts is accompanied by a decrease in the bias of the spring with respect to` the binfetal blade, said bias decreasing at a rate greater than the rate of decrease of the reaction of the bimetal blade on the spring whereby the contacts are opened with an accelerated movement. It is also true that the relation between the parts is such that the approach of one contact toward the other is accompanied by an increase in bias of the control spring with respect to the bimetal blade, said increase taking place at a rate greater than the rate of increase of the reaction of the blade against the springloaded arm whereby the reengagement of the contacts takes place at an accelerated rate. Therefore when once the contacts begin to separate, they will separate with a quick, abrupt motion and also when the contacts begin to move one toward the other, the movement is relatively rapid. Consequently, the switch operates with an abrupt or snap action, tending to minimize sparking at the contacts.

While the embodiment of the present invention constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

1. A switch comprising in combination, a base,

a stationary contact secured to the base; a bimetallic thermal responsive element anchored at one end and having the other end adapted to flex in response to a temperature change; a cooperating contact secured to the thermal responsive element in alignment for engagement with the firstmentioned contact; a tension spring anchored at o ne end; a link member pivotally connected to the thermal responsive element and having the other end of the tension spring connected thereto; said tension spring, link and thermal responsive element being so relatively angularly disposed that a component of the force of said tension spring biases the contacts toward engagement .by an amount which diminishes as the contacts move away from enga'gement; and stop means for preventing the said bias from passing through a zero value.

2. A switch comprising, in combination, a base,

a stationary contact secured to the base; a bimetallic thermal responsive element anchored at one end and having the other end adapted to flex in response to a temperature change; a cooperating contact secured to the thermal responsive element in alignment for` engagement with thev first mentioned contact; a tension spring having one end anchored near the anchored end of the thermal responsive element and extending be- 'yond the free end of the thermal responsive element; means pivotally engaging the thermal element and angularly disposed with respect to the axis 'of the spring and the thermal responsive element for transmitting a component of the force of the spring to the thermal responsive element to normally bias the thermal responsive element in one direction; and stop means for preventing said means from crossing the axis of the spring' 3. A switch according to claim 1 in which the stationary contact is yieldably mounted so as' to` 'move with the contact on the thermal element as 'the latter moves in a direction/to separate the 

